There is a variety of applications in which it is useful to be able to provide a two-dimensional map of the distribution of charge or of potential across a surface. This map is normally in electronic form for use by a computer for processing of the information in the map or for storage in a memory medium of some sort.
Many of these applications involve the storing or processing of images such as writings, drawings, and photographs. etc. Typically, an optical image of the writing, drawing, picture, signature, etc. is transformed by a lens to fall onto an array of photodetectors which then provides an electronic description of the intensity of the light falling onto the individual elements of the photodetector array.
Other applications involve the creation of such images or other patterns via stylus input onto an electronic tablet or screen or via the sensing of fingerprints by means of a pressure-sensitive surface or directly by charge variations on the skin surface that correspond to the fingerprint. These images or patterns are not sensed optically but are, instead, sensed by charge or field inputs that are applied to selected areas of a sensing array. Such images and patterns may also be created by the measurement of thermal variations.
These applications are presently being met in the art by a variety of different devices, but most of them require significant amounts of power and are unable to store a sensed state without the further application of power to refresh their associated electronic memory storage units.
The methods currently used to perform these tasks may be costly in both time and material. For example, the recognition of an image usually requires some scanning system where the image is swept or scanned optically. The signal corresponding to pixels of the image detected by a photosensitive detector is then transformed into a digitized image. Certain applications that involve heavy use by the public, such as reading the finger print of a client at a point of sale for comparison to an electronic image of a finger print stored on a checking card ("smart card") will require devices that are rugged and inexpensive. Similar demands are set on all applications where one is required to install large numbers of such sensing and identification devices in circumstances such as those found involving charging the cost of a telephone call from a public phone via a smart card, identifying passengers in transit in air terminals, transactions at ATM machines, identification at security checkpoints and the like.
It would therefor be exceedingly useful if a sensing device existed that could perform these various applications and that had at least some of the following characteristics: simple and robust in construction, low power, low cost, readily portable as necessary and non-volatile. No such device is known at this time however.